1. Field of the Invention
This invention is in the field of pressure sensitive carbonless copying sheets including a barrier layer between a groundwood containing base sheet and an overlying color forming layer, the barrier layer containing an alkali metal sulfite to prevent yellowing of the sheet.
2. Description of the Prior Art
The use of carbonless pressure sensitive copying sheets has become very widespread in recent years. Typically, such pressure sensitive sheets are sold as a manifold paper pack in which the top sheet consists of coated back (CB) paper having a micro-encapsulated coloring coating on its back, an intermediate sheet of paper which is coated front and back (CFB), having a color former reactive with the coloring coating on its front side and a color forming coating on the back, and a third sheet of coated front (CF) paper containing a color reactant on the front but no coating on the back.
The patented art describing carbonless pressure sensitive copying sheets is quite extensive and the following citations of such prior art are purely representative and by no means all inclusive.
Hemstock U.S. Pat. No. 3,223,546 describes sensitive sheets in which the coating consists of an encapsulated color forming organic material such as crystal violet lactone or benzoyl lueco methylene blue and the color reactant is a calcined kaoline clay which has been treated hydrothermally to transform it into an amorphous aluminum silicate.
Harbort U.S. Pat. No. 3,293,060 describes a pressure sensitive copy sheet in which the color reaction takes place between a spiro-dipran and an acid activated clay.
Amano et al. U.S. Pat. No. 3,330,722 describes a paper which is filled with clay and a water insoluble salt of cobalt, manganese, lead or mixtures thereof, such coating being capable of color reaction with various organic compounds.
Kan et al. U.S. Pat. No. 3,432,327 contains a very comprehensive disclosure of pressure sensitive copying sheets and combinations of materials which will react with each other to produce a colored mark in the presence of hydrophilic liquid.
Kimura et al. U.S. Pat. No. 3,506,471 provides a coloring agent in the microcapsules consisting of a fluorane derivative.
Brockett U.S. Pat. No. 3,516,845 describes paper coating compositions for pressure sensitive record sheets which contain kaolin, phenolic resins, and salts of metals such as zinc chloride. The addition of such salts is said to enhance the color activation by kaolin-phenolic resins and the face resistance of the colored form of the normally colorless color forming materials.
Watanabe U.S. Pat. No. 3,535,139 describes a color forming system in which one of the color formers is a derivative of a polyhydric phenol substantially insoluble in water and the other is an iron compound or vanadium compound.
Kimura et al. U.S. Pat. No. 3,551,181 describes a pressure sensitive copying paper in which the rupturable microcapsules contain an oil in which a phthalein compound is dissolved.
Philips U.S. Pat. No. 3,558,341 describes encapsulated color formers in which the effective ingredients are symmetrical N' disubstituted dithiooxamides.
Brockett U.S. Pat. No. 3,634,121 refers to a sensitized sheet record material containing fine mineral particles having absorbed thereon acid-reacting oil soluble polymeric materials such as phenolic polymers.
Miller et al. U.S. Pat. No. 3,663,256 describes a color forming system in which one of the color forming reactants is a phenolic resin containing at least 70% by weight of an oil soluble thermoplastic polymeric condensate of formaldehyde and at least one p-substituted phenol, the other color former being material such as crystal violet lactone.
Hayashi et al. U.S. Pat. No. 3,682,680 refers to a clay coated sheet for pressure sensitive copying sheets, wherein undesirable color changes such as yellowing and decreased activity are prevented by incorporating an organic acid salt into the clay-coated layer thereof.
Of particular interest in regard to the present invention is Ruus U.S. Pat. No. 3,429,827, the disclsoure of which is incorporated herein by reference. This patent describes an improved manifold paper having microscopic pressure rupturable capsules on at least one surface thereof, each capsule shell consisting of a high molecular weight condensation polymer such as a polyamide. The preferred method for the manufacture of the encapsulated particles is to disperse or emulsify one reactant for the condensation polymer in a continuous phase containing the second reactant. The substance to be encapsulated, the color former, is also contained in the dispersed phase. The polycondensation polymer shell forms at the inner phase of the dispersed substance and encapsulates the material. A typical reaction to form the polyamide shell results from the condensation of a polyhydric alcohol with teraphthaloyl chloride and a polyalkylene amine.
It would be highly desirable, particularly for reasons of economy to use paper sheets in the manifold which contain significant amounts of groundwood. It has been found, however, that where a groundwood sheet is contacted with a color former which is encapsulated in polyamide microspheres, the sheet turns yellow as the coating dries. Presumably, this is due to excess amine present in the coating which reacts with the lignin of the groundwood sheet to cause a discoloration.